To allow examination of the structure of biological samples such as tissues or cells using an electron microscope or high-resolution optical microscope methods, ultrathin sections (only a few nm thick) are prepared and are applied onto specimen carrier grids (hereinafter referred to as metal grids) made of metal, preferably of nickel. For microscopic examination the sections are contrasted, or individual constituents of the sample are labeled using special cytochemical methods. These cytochemical methods are often based on the principle of ligand pair formation: a first ligand can be contained in the biological sample, and the second ligand, when it comes into contact with that sample, then binds as a binding partner to the first ligand. Examples of biologically based ligand pairs include antigen/antibody binding pairs, enzyme/substrate binding pairs, lectins/sugars, hormone/receptor systems, and DNA/DNA and DNA/RNA pairs.
Numerous methods involving the antigen/antibody binding pair are known in the existing art; these are grouped under the heading of immunohistochemistry and immunocytochemistry (hereinafter referred to as immunological labeling techniques). U.S. Pat. No. 5,143,714, for example, discloses a method that adsorbs an antigen out of a liquid sample in a pelletizable gel substance. The gel pellet is surrounded by a diffusion barrier, integrated as a block into a stamped-out gel matrix, and subsequently subjected to immunological labeling techniques just like a tissue sample. German Pat. No. DE 38 78 167 T2 describes the use of colloidal gold particles to label ligands, using the immunogold staining technique. A greatly improved method that permits qualitative and quantitative evaluation of an antigen in a sample is disclosed in U.S. Pat. No. 5,079,172 in the form of a sandwich assay, in which the first antibody that binds the antigen is labeled with a gold-labeled second antibody that binds the first antibody. Using electron-microscopy evaluation methods, the antigen the sample can be determined qualitatively and quantitatively based on the quantity of gold particles.
A characteristic shared by many immunohistochemical and immunocytochemical protocols for immunological labeling of thin tissue sections is the fact that they usually comprise ten to 20 individual process steps. Most of the process steps comprise operations in which the sample under examination is washed with a buffer solution or labeling solution.
At present, these washing operations are performed manually, in a laborious process in which individual droplets of the aqueous buffer solution or labeling solution are applied with a pipette onto a hydrophobic substrate (e.g. Parafilm®, Parlodion®, Colloidion, or Formfan®). The metal grids with the thin tissue sections are individually laid down thereon in order to react with the treatment liquid. Because of the light weight of the metal grid and the surface tension of the liquid droplet, the metal grid floats on the droplet surface. After a certain residence time for this step (often 5 to 10 min), the metal grid is transported with tweezers to the next droplet. This operation is continued up to the last position of the standard protocol, and occupies a technician for as long as several hours for each immunological labeling reaction.
It is readily apparent that this manual process requires constant attention by the operator, and entails high labor costs because of the large time requirement. The number of samples to be processed simultaneously is greatly limited, and errors by the operator while precisely pipetting and positioning liquid droplets with very small volumes cannot be excluded. The manual method cannot rule out confusion of samples after the long processing time during immunological labeling; this could be prevented by using a sample carrier having an identifier in the form of a chip or barcode, as presented in German Utility Model DE 299 06 382 U1.
In addition, evaporation of the liquid droplets during longer-duration standard protocols constitutes a major problem.
Although German Utility Model DE 298 17 912 U1 discloses an apparatus for washing microscopable preparations on supports after immunocytochemical treatment, it refers to a wash box in which a larger quantity of washing solution flows through at a certain flow rate over the preparation and carrier. This apparatus is not suitable for the implementation of immunological labeling techniques themselves, since the antibody-containing labeling solutions that are used are very expensive and are therefore employed in only the smallest possible volumes.
An apparatus and method that implement in fully automatic fashion the operation of carrying out immunological labeling techniques for thin tissue samples are not presently known.